1. Field of the Invention
The present invention relates to a fiber optic interferometer and a method for determining physical state parameters in the interior of the coil of a fiber optic interferometer.
2. Description of the Prior Art
Fiber optic interferometers are employed, inter alia, for measuring rotational speeds. To accomplish this, a light beam is split into two partial beams, guided in opposite directions in a circle, to meet one another again after one or more circulations. The interference pattern of the two superimposed partial beams changes if the fiber optic interferometer is rotated about an axis perpendicular to the beam plane, as the optical paths of the two partial beams are no longer of equal lengths.
The resulting interference pattern is not only a function of rotational speed, but may also depend on external parameters such as temperature. Since fiber optic interferometers are employed worldwide today over a wide range of applications, they must possess consistent operating behavior under demanding conditions (e.g., over a temperature range of −55° C. to 90° C.). This is achieved by the use of individual sensors calibrated over the operationally-required temperature range.
During calibration, fiber optic interferometer output signals, a function of the phase shift of the two light beams traveling in opposite directions, are detected with known external parameters. Thereafter, a suitable mathematical model is determined as a function of an input parameter (e.g., the currently prevailing ambient temperature) and stored in the memory of the fiber optic interferometer signal processor. During operation, the measured data are then evaluated as a function of the input parameter. This can only represent an approximation, however, as the properties of the fiber optic interferometer are dependant upon not only a single input parameter such as temperature, but on a multiplicity of parameters which (to a first approximation) are physically independent of or interact with one another (e.g., temperature and expansion due to temperature-dictated change in length of the fiber). The properties of the fiber optic interferometer also depend on temporal profiles of temperature at a given location (i.e. temperature transients) or at different locations at a given time (i.e. temperature gradients). Using the example of temperature, this can be explained in greater detail in a simple way: the propagation time of light in the core of the optical fiber is dependent on the length of the optical fiber and such length changes as a result of material expansion due to temperature increase. It is also dependent on the refractive index n within the optical fiber. The refractive index depends in turn both directly (due to the material property of the fiber) and indirectly on temperature as a change in length of the fiber due to temperature is accompanied by varying, spatially locally distributed mechanical stresses σ in the inner fiber core of the wound fiber that can lead to a change in refractive index n. When local mechanical stresses on the fiber are intensified, cross-couplings between the fast and the slow axes of a polarization-maintaining optical fiber can also occur that result in considerable changes in propagation time. For precise calibration of a fiber optic interferometer, it is thus desirable to obtain metrological access to the state parameters at the interior of the optical fiber using conventional measurement methods. A calibration preceding operation of the sensor has the disadvantage that the conditions under which the sensor is intended to be used must be previously defined. The calibration is then only valid within such previously-defined conditions.
In order to determine the temperature-dependent behavior of a fiber optic interferometer, it is known to use one or more temperature sensors, usually based on semiconductors, individually or simultaneously in an assemblage. They are often arranged in the vicinity of the interferometer, the phase modulator or directly on the topmost layer of the optical fibers wound to form a coil. In such an arrangement, however, it is not possible to directly measure the instantaneous state at the interior of the optical fiber. Thus, the measured temperature is not necessarily that at the interior of the optical fiber as complex conditions of heat transport from the surroundings of the fiber into the interior of the fiber arise due to the different heat capacities of the materials used in the fiber optic interferometer. An external change in ambient temperature will have an effect some time later in the interior of the optical fiber. Such time is temporally and spatially dependent on many, mainly unknown, parameters and, consequently, can only be determined to an approximation. Additionally, an identical temperature does not necessarily produce the same propagation time of the light beam. For example, as a result of hysteresis, should a specific temperature be established after a cooling or heating process, other internal states can be established with the same temperature conditions at the core of the optical fiber. In that case, the same measured temperature is then present and there is compensation to the same extent in both cases, even though the actual propagation times of the light beams are different.
A possible method for eliminating such disadvantages might be, for example, to wind conventional semiconductor temperature sensors and/or pressure sensors together with the optical fiber to form the fiber coil. Technical procedural difficulties would occur, however, when winding the optical fiber, such as the rebound of the optical fiber as a result of bulges within the coil at the locations of the sensors. The sensors within the coil could additionally produce local changes in mechanical stresses in the optical fibers wound below and/or above them, that, in turn, would influence the profile of light propagation times due to the influence of mechanical stresses on the refractive index of the fiber core material.